We will: 1) develop new approaches for the treatment of diarrheal diseases by using drugs to increase intestinal absorption or to decrease intestinal secretion; 2) identify phosphorylated protein intermediates involved in cyclic nucleotide-induced stimulation of active intestinal electrolyte secretion and inhibition of NaCl coupled absorption; 3) establish a functional role for these phosphorylated intermediates by demonstrating: a) a correlation between changes in the phosphorylation of the intermediates and the cyclic nucleotide-induced changes in active electrolyte transport and b) a correlation between drug-induced changes in the phosphorylated intermediates and drug-induced changes in cyclic nucleotide-induced electrolyte transport. The methods to be used to study water and electrolyte transport, the enzymes involved and the phosphorylated intermediates will include 1) in vivo the single pass perfusion technique; 2) in vitro the Ussing chamber and voltage clamp technique combined with ion flux measurements using radioactive tracers; 3) measurement of the activities of the intestinal mucosal enzymes: Na-K-ATPase, Mg-ATPase, adenylate cyclase, guanylate cyclase, cyclic nucleotide phosphodiesterase, protein kinase and phosphoprotein phosphatase; 4) measurement of intracellular concentrations of the cyclic nucleotides cAMP and cGMP; 5) peptide separation by one and two dimensional slabgel electrophoresis of purified intestinal cell plasma membranes and other cell organelles; and 6) quantitation of phosphorylation of individual peptides by autoradiography and densitometry.